Question About Action Potentials

[Mission Medical]

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After the hyperpolarized part of the action potential is reached, what exactly happens next? How does the membrane potential raise back to resting potential? Also, since at this point we have brought lots of Na+ into the cell and pushed lots of K+ out of the cell, how do we reverse this? Does the Na+/K+-ATPase pump take care of this?

 

[mehc012]

After the hyperpolarized part of the action potential is reached, what exactly happens next? How does the membrane potential raise back to resting potential? Also, since at this point we have brought lots of Na+ into the cell and pushed lots of K+ out of the cell, how do we reverse this? Does the Na+/K+-ATPase pump take care of this?

The hyperpolarization is due to the increased permeability of the membrane to K+ during the falling phase, which drives the membrane potential closer to the equilibrium potential of potassium (~-80mV). Once the cell repolarizes, however, the voltage-gated K+ channels close, and the sodium potassium pump (which has been running the entire time, remember) brings it back to the equilibrium potential.

Keep in mind that, though we often represent these shifts as extreme changes in concentration, it actually doesn't take much of a concentration shift to alter the membrane potential...or to restore equilibrium. Active transport can restore things pretty quickly.